Thanks a lot, guys!

As I said, I actually didn't mean to use a different background than the standard dark blue from L2 shadow. But that one doesn't seem to be part of the background object list; it counts as "no background", with this shade of blue merely being the standard colour scheme for L2 shadow.
But since I had to pick one, once I decided for this one, I just ran with it - notice the three trees in the background standing on the snowy hill - true physics mode reveals they're actually floating in the air!

@Colorful Arty: Thanks - now I finally know how to spell that word that you keep using in your LPs!

To me, it always sounded like "dog on it"...
And btw, the main reason I can make these levels look pretty is because the tiles in and of themselves already look decent.

My entirely custom graphic sets look a lot more "rough" in comparison!
@Raymanni: Atmosphere was precisely what I was going for.

Less flashy red and green lights, like on the actual Christmas tileset, and more "Silent Night"...
Might be an inspiration to people who create whole packs for the Holiday season, perhaps? We have quite a few graphic sets containing snow by now, so not all levels need to feature the same standard Christmas tileset in order to go with the theme...